The present invention relates, in general, to the process of making thin film depositions on a substrate and, more specifically, to a process for controlling the grain structure growth of a thin metal film after having been deposited on a substrate.
A large variety of thin films are used in the fabrication of very large scale integrated circuit devices. These films may be thermally grown or deposited on a substrate. The thin films may be metals, semiconductors, or insulators.
There are several techniques for depositing films on a substrate. One such technique may be performed in a vacuum chamber and is known as physical vapor deposition or sputtering. Another technique may be performed in a bath and is known as electroplating.
It is known that sputter deposited copper films have a characteristic as-deposited microstructure which changes as a function of time at room temperature. This phenomenon has been documented by J. W. Patten et al. in xe2x80x9cRoom Temperature Recrystallization in Thick Bias Sputtered Copper Deposits,xe2x80x9d Journal of Applied Physics, vol. 42, No. 11, pages 4371-77 (October 1971).
Work has shown that, for electroplated copper, the as-plated copper has a fine-grained microstructure, with an average crystallite size of less than 100 nanometers. Verification of this microstructure in the as-plated film has been established using Back-Scattered Kikuchi Diffraction (BKD). When stored at room temperature, no change has been observed in the fine-grained microstructure for a period of 8-10 hours; this time period is known as the incubation period. After the incubation period, grain growth has been observed for the next 10-20 hours, with the microstructure then reaching an apparent steady-state having an equilibrium structure.
In order to take advantage of the fine-grained microstructure of copper, certain critical process steps must be carried out within 20 hours after copper electroplating. This requirement is difficult to meet in a manufacturing environment in which, for example, the as-deposited substrate may sit on a shelf over the weekend.
It is also known that grain growth in the microstructure of copper may be accomplished within the space of several minutes by heating the copper at a high temperature. Heating a metal to change its microstructure is an established metallurgical practice. It has been believed, however, that for a metal such as bulk copper, a relatively high temperature of at least 350xc2x0 C. is required to obtain any appreciable change in its microstructure. This has been documented in xe2x80x9cMetals Handbook,xe2x80x9d Vol. 4, pages 719-28 (9th ed., American Society for Metals, Metals Park, Ohio, 1981).
Several semiconductor manufacturers are replacing the current aluminum interconnect metallization with copper wiring, because copper offers superior electrical conductivity and electromigration performance. Several deposition techniques are possible, one of these being electroplating. Electroplating has advantages: it has excellent trench fill properties and produces a copper film with near zero residual stress.
Electroplated copper interconnects may be used in multi-chip modules for both power distribution and signal transmission. In the more complex structures, multiple levels of wiring may be required. The fabrication of these wiring levels is well known in the art. The grain structure of the plated copper is critical. If the plated copper has a fine-grained microstructure, the etching results in a smooth surface. If the plated copper has a large-grained microstructure, however, the etching results in a rough surface. The rough surface has a disadvantage because such a surface precludes an accurate measure of the thickness of the polyimide layer which is deposited over the wiring.
The deficiencies of the conventional processes used to deposit thin metal films on a substrate show that a need still exists for a process which can control the grain structure growth of a thin metal film after the film has been deposited on a substrate.
To meet this and other needs, and in view of its purposes, the present invention provides a process of depositing thin metal films having different microstructures onto a substrate. The process controls grain growth in the microstructures and, in one embodiment, includes the following steps:
(a) a metal film is deposited onto the substrate to form a film having a fine-grained microstructure, and
(b) the metal film is heated in a temperature range of 70-100xc2x0 C., for at least five minutes, to convert the fine-grained microstructure into a stable, large-grained microstructure.
In another embodiment, the metal film is frozen, after the step (a) of depositing, at a temperature not greater than xe2x88x9220xc2x0 C., wherein the fine-grained microstructure is stabilized without grain growth for the entire freezing period.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.